1. Field of the Invention
The present invention relates to molding articles from expanded foam material, and more particularly to an apparatus and method for making foamed boards or sheets.
2. Description of the Prior Art
Foam material, such as foamed polystyrene, possesses low heat transfer characteristics and other desirable properties because of its cellular structure. As a result, it finds use in a wide variety of applications, including use as an insulating material in the construction field. For such use it is desirable to employ large sheets of foam material having the same size as the plywood sheets, gypsum board sheets, and other materials used in the building trades.
Several types of machines have been developed for manufacturing or molding foam boards and reference should be made to U.S. Pat. No. 3,042,967 for a description of basic foaming techniques. One such type of machine typically has a stationary frame member forming a molding cavity of appropriate dimensions. A cover fits over the cavity and is locked on the stationary frame member. Thereafter, the cavity is filled with partially expanded beads of polystyrene and steam is supplied to the cavity to expand the beads to their completely foamed state. The foam is then stabilized to form a board by cooling it with water. Finally, the board is ejected by a plurality of knock-out plungers from the mold cavity and the machine recycled. The foam material boards produced are typically 4'.times.8' sheets having a thickness ranging from about 1" to 6".
Another type of molding machine is shown in U.S. Pat. No. 4,272,469. This machine includes a pair of platens fixedly mounted on a frame, and a second pair of platens which are movable toward and away from the stationary platens. In this manner the volume of the mold cavities may be varied to provide the desired board thickness.
Numerous problems have arisen in connection with machines currently in use. The heat of the steam used to expand the beads also expands the metal components of the machine. The design and construction of current machines is such that such expansion interferes with the operation of the machine or generates stresses leading to damage or failure of machine components. Thermally induced alterations of the machine and its components may also adversely effect the quality of the board. For example, the surfaces of the mold cavity may become wavy as they expand producing a board that is similarly uneven.
The forces produced by the steam in the cavity and the release of the blowing agent from the styrene beads may easily exceed 100,000 lbs., similarly causing misalignment, deflection, or fracture of various components of the machine. The so-called "lock-ups" that secure the cover to the stationary frame of many machines are particularly subject to damage or failure in resisting or relieving these pressures.
In the molding process, it is conventional to initially pre-expand the raw polymeric particles or "beads" by the application of heat to form "pre-puffs" prior to their further expansion in a molding cavity into the final board. It has been found that pre-expansion of the beads results in a more uniform final expansion, better fusion of the foam, the elimination of voids in the foam, and lower molding pressures. In addition, it has been found that the density of the final board may be more readily controlled by controlling the density of the pre-puffs. Although many pre-expansion techniques have been developed, it is extremely difficult to reproduce identical physical characteristics, especially density, in the beads from batch to batch so that a uniform final board may consistently be manufactured. This poor expansion control is the result of variances in the raw polymer material itself, the content of the blowing agent used with the bead, and the moisture in the environment. Thus, it is desirable to eliminate the need for precise pre-expansion controls.
A molding machine must also be capable of high production cycles for economic reasons. In order to accomplish this, the cycle time between filling the mold cavity and ejecting a finished board must be kept at a minimum. In the past, machine operators have determined an optimum cycle time, and in particular the cooling period, by a trial and error technique. The optimum cooling time must be determined so that the board is properly cured or dimensionally stable when ejected, and so that unnecessary cooling is eliminated which conserves utilities and reduces manufacturing costs. If not cooled enough, the center of the board will continue to expand after ejection from the mold causing it to take on a bloated configuration. On the other hand, if cooled too long unnecessary water usage occurs raising costs, and cycle times increase reducing production rates. It is thus desirable to provide a molding machine with means for eliminating the guesswork in determining the optimum molding cycle.
Poor cooling techniques have also been a problem. Air pockets may form which prevent cooling water from reaching certain surfaces of the board, especially its corners, resulting in the formation of a non-uniform skin and scorching of the board surfaces. It is thus desirable to provide a molding machine with means for preventing corner air pockets from forming to provide uniform cooling of the board.
Another problem involves removal of the board from the mold cavity once the board has been formed. The board is often wedged in the mold cavity because of the expansion of the styrene beads. As the board is pushed from the cavity by the plungers it may become arched and fly out uncontrollably, or the plungers used to eject the board from the cavity may puncture the board.